1. Field of the Invention
This invention generally relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having improved observation angle characteristics.
2. Description of the Related Art
Remarkable improvements have been made in liquid crystal display devices, and the liquid crystal display devices themselves have gained wider general applications. Nevertheless, demands for a higher accuracy of the liquid crystal display devices and for a greater display screen sizes have increased, as have demands for an improvement in the display quality, and improvements in the observation angle performance, for example, are also required.
Conventional liquid crystal display devices in general have the structure shown in FIG. 23 of the accompanying drawings, which is an exploded perspective view showing a structural example of a typical conventional liquid crystal display device, which is a simple matrix type liquid crystal display device.
In the drawing, reference numeral 10 denotes a liquid crystal cell. This cell is produced by, for example, disposing two laminate structures of a stripe-like drive electrode consisting of an ITO (In.sub.2 O.sub.3 - SnO.sub.2) film, for example, and an alignment layer consisting of a polyimide resin film, between transparent substrates 1,1 consisting of a glass plate, for example, and charging and sealing a liquid crystal 5 into the space defined therebetween. The direction of a dashed-line arrow in the drawing indicates the direction of orientation treatment of the alignment layer on each substrate, e.g., the rubbing direction. The rubbing directions are usually set so as to orthogonally cross each other, in the case of a TN type liquid crystal display panel. Reference numerals 7a and 7b denote polarization plates, respectively, that are disposed on both sides of the liquid crystal cell 10 in such a manner that the directions of the optical axes thereof orthogonally cross in the direction indicated by the solid line arrow in the drawing. Note, in the drawing, symbol .phi. represents a rotating angle of the observation angle, with a line X--X as the reference, and .theta. represents an elevation of the observation angle from line Z--Z. These are shown in order to indicate the directions of the observation angles.
Generally, the liquid crystal display device is operated in the following way. Rays of light are irradiated from the lower part of the drawing, and the light is converted to linearly polarized light by the polarizer 7b and made incident on the liquid crystal cell 10. When an electric field is not applied to the drive electrodes disposed so as to orthogonally cross each other, the polarization plane of the linearly polarized light is rotated by 90.degree. and the light passes through the upper analyzer 7a; i.e., the display at this time is ON (bright). When the electric field is applied to a desired point of intersection of the drive electrodes, the orientation of the liquid crystal molecules at that portion changes, so that the transmitted light passes through the liquid crystal cell 10 without rotating the polarization plane and is blocked by the upper analyzer 7a; i.e., the display at this time is OFF (dark).
Accordingly, a bright and dark image display is effected by controlling the ON-OFF of the voltage applied for each pixel, by a drive control circuit not shown in the drawing.
FIG. 24 is a schematic view showing the orientation state of the liquid crystal molecules in a conventional liquid crystal display device, and shows the case of a liquid crystal cell which exhibits a "homegeneous orientation", such as a TN type, an STN type, etc. Reference numeral 3 in the drawing denotes an alignment layer, and 50 denotes the liquid crystal molecules.
FIG. 24 (A) shows an example where the electric field is not applied, and FIG. 24(B) shows the case where the electric field is applied. When the electric field is not applied, a twist of the liquid crystal molecules by 90.degree., for example, occurs between the upper and lower substrates, due to the orientation limiting force of the alignment layer 3. When the electric field is applied, it can be seen that the liquid crystal molecules rise perpendicularly to the substrate surface. Therefore, the ON-OFF control of the liquid crystal display device is effected as described above.
FIG. 25 shows transmission factor-v-applied voltage characteristics in a conventional liquid crystal display device. The transmission factor is plotted on the ordinate and the applied voltage on the abscissa. In the diagram, the solid line 1 represents the case where .theta.=0, i.e., where the liquid crystal display panel is viewed from immediately above, the dashed line 2 represents the case where .phi.=90.degree., i.e., where the liquid crystal display panel is observed from the deep side, and the dotted line 3 represents the case where .phi.=270.degree., i.e., where the panel is observed from the front side.
Even when the electric field to be applied is uniformly increased, as in the example given above, the transmission factor, and thus the contrast, differ remarkably depending on the direction of the observation angle, and further, a maximum value M (inversion of displayed image) will occur in some cases.
Such an inversion of the displayed image occurs because the orientation direction of the liquid crystal molecules 50 appears different depending on the direction from which it is observed, as shown in FIG. 24(B), and thus the polarization state of the transmitted light does not change in the same way.
The inversion of the display leads to a serious problem in the display performance of the liquid crystal display devices, particularly liquid crystal display devices of a large screen size and high performance, is lowered, and thus a solution to this problem must be found.